1. Field of the Invention
This invention relates to novel negative photoresist compositions and to the production of negative photoresist films. In particular, the invention deals with a novel class of copolymers of .alpha.-chlorovinyl methyl ketone. When cast in thin films and exposed to radiation (such as ultraviolet light), these copolymers form a crosslinked network making the materials useful as negative photoresists.
2. Description of the Prior Art
Photoresists are materials which change their solubility in response to a developer solution after the photoresist has been exposed, such as to ultraviolet radiation. Photoresist compositions may comprise a photo-sensitive compound (hereafter sometimes called sensitizer of photosensitizer), which is blended with a film forming polymeric resin and a solvent. Photoresist compositions may comprise also polymeric materials which of themselves are inherently light sensitive. It is with the latter type, specifically a copolymeric material, that the present invention is concerned. As a consequence of the exposure to radiation of the photoresist, a different solubility rate results between the exposed and unexposed (masked over) portions of a resist film that yields a surface relief pattern after the development. Those photoresists which become more soluble in the exposed regions are referred to as positive photoresists.
The photoresists are applied in any suitable manner, such as by spin coating from an organic solvent or solvent mixture, onto a substrate, such as silicon wafers and chrome plated glass plates. A developer removes the areas of the coated photoresist film that has been exposed to light or other form or irradiation so as to produce a pattern in the photoresist film that has been exposed to light or other form of irradiation so as to produce a pattern in the photoresist film.
The application of the photosensitive film to various substrates is an essential step in the fabrication of integrated circuits. The substrates are generally silicon wafers which may have a thin oxide coating or other coating such as a silicon nitride or aluminum. The photosensitive film is used to pattern the substrate in a series of steps including exposure, development and substrate etch. It is essential that the mask pattern be accurately reproduced in the substrate etch pattern. To achieve this high degree of accuracy, it is essential that the photoresist film be of uniform thickness, have good adhesion to substrates, good contrast in images formed, and good etch resistance properties.
One of the limitations of materials currently used as commercial photoresists is their lack of sensitivity to light at wavelengths much below 300 nm. The resolution attainable with these resists is typically in the 2.mu.-4.mu. range and would be improved were they sensitive to shorter wavelengths of light. There is, therefore, a need for photoresist materials that are sensitive in the deep ultraviolet (UV) light range (200 nm-300 nm), in that
a higher resolution and thus a such materials permit a higher resolution and thus a higher information density in the microcircuits manufactured using them. This sensitivity in the deep UV light range should of course be accompanied by the other normal attributes of a resist material including good adhesion to substrates, good contrast in images formed, good etch resistance properties, and the like.